1. Field of the Invention
The present invention relates to a NOx-decomposing electrode, i.e., an electrode for decomposing or reducing NOx, especially for decomposing NOx to produce oxygen, and to a NOx concentration-measuring apparatus for measuring NOx contained in the atmospheric air or in the exhaust gas discharged from vehicles or automobiles.
2. Description of the Related Art
NOx contained in an exhaust gas discharged from a vehicle or an automobile is measured. In a conventional measuring method, when an exhaust gas discharged from a vehicle or an automobile is introduced as a measurement gas into a sensor which includes a NOx-decomposing electrode, NOx contained in the measurement gas is measured by measuring an electromotive force generated on the NOx-decomposing electrode (see Japanese Laid-Open Patent Publication No. 11-183434). In this method, the NOx-decomposing electrode is a cermet electrode composed of an alloy of Pt—Rh and a ceramic component. The NOx-decomposing electrode is formed on an oxygen ion-conductive solid electrolyte such as zirconia.
The ratio between Pt and Rh (Pt:Rh) in the NOx-decomposing electrode ranges from 10:90 to 99:1 or from 25:75 to 75:25 by weight. When such a ratio is adopted, the oxidation reaction and the reduction reaction of Rh are suppressed on the NOx-decomposing electrode. Further, even if the NOx-decomposing electrode manufactured by the ratio as described above is used for a long period of time, the contact area between the NOx-decomposing electrode and the solid electrolyte is not changed. Accordingly, the impedance of the pumping cell provided in the sensor is not increased. Therefore, a NOx concentration-measuring apparatus using the NOx-decomposing electrode as described above realizes the stabilization of the impedance of the pumping cell and the stabilization of the sensitivity of measurement of NOx.
In such a NOx-decomposing electrode, the ability to decompose NOx is required to be high, and the oxidation reaction and the reduction reaction should be suppressed. The ability to decompose NOx is improved by making the ratio of Rh in the Pt—Rh alloy larger than that of Pt. On the other hand, the oxidation reaction and the reduction reaction are suppressed by making the ratio of Pt in the Pt—Rh alloy larger than that of Rh. Therefore, it is impossible to realize the improvement of the ability to decompose NOx and the suppression of the oxidation reaction and the reduction reaction in the NOx-decomposing electrode as described above.
Further, the NOx-decomposing electrode measures NOx at a high temperature from 700° C. to 800° C. Therefore, the NOx-decomposing electrode repeats the expansion and the contraction every time when NOx is measured. As a result, the NOx-decomposing electrode is exfoliated from the solid electrolyte, and the ability of the NOx concentration-measuring apparatus to measure NOx is lowered.